The present invention relates to an improvement in a storke control system for an impact device which impact device is driven by a compressible fluid and is capable of performing a series of blows. Such an impact device, or hammer, is described in U.S. Pat. No. 3,464,315 to which reference is made. U.S. Pat. No. 3,818,799 discloses a control system whereby the number and intensity of a series of blows to be performed by such a hammer may be pre-programmed thus permitting essentially automatic operation. This control system utilizes a switch which is adjustably positionable on the frame member and actuated by the moving hammer to sense the position of the ram and thereby properly coordinate the operation of the main driving fluid valve. The length of time said valve remains open together with the supply pressure determines the intensity of the impact of successive blows. The velocity of the ram varies depending upon the characteristics of the forging blow. Therefore, timing the opening of the main driving fluid valve to drive the ram the same way on each successive stroke has produced variable force results at the next impact. Thus, in the prior art, despite the use of sophisticated programming controls, successive blows have been variable and of unpredictable intensity.
We have determined that the causes of this variability has been either the variation in the amount the fluid supply port has been opened, or the variation in the degree of piston stroke before the supply valve is opened. To the extent that the port has been partially closed by the piston when the fluid supply valve has been opened, there has been a decrease in the effective force of the hammer blow. To the extent the piston stroke is greater when the supply valve is opened, there has been a decrease in the effective force of the blow.
The present invention provides a means to assure that the port is not partially blocked by the piston and the piston stroke is consistent where the supply valve is opened. Therefore, the intensity of a blow cannot be modified by a partially occluded port or variable piston storke. In particular, the timing of the opening of the main driving fluid valve is such that the piston is properly oriented to assure the port is just completely open on each successive blow.